For the purpose of e.g. improvement of fuel consumption efficiency, a vehicular engine is subject to a control for effecting a warm-up operation when the temperature of the engine is low and maintaining the temperature constant after the engine temperature has risen. To this end, as a cooling system for an engine, there is generally known a system configured as follows. With opening/closing of a thermostat valve, when the temperature of cooling water is low, this valve is closed to cause the cooling water to circulate via a bypass flow channel, bypassing a radiator; whereas, when the cooling water temperature has risen, the valve is opened to cause the cooling water to flow through the radiator, thus maintaining the cooling water temperature constant. Further, under a low temperature condition of the cooling water, via a warm-up operation, the engine temperature is raised speedily to the optimal temperature. Thereafter, the engine temperature is maintained approximately constant to stabilize combustion for improvement of fuel efficiency.
Patent Document 1 discloses a thermostat wherein a channel to a radiator exit channel is provided in a thermo element temperature-sensitive chamber provided independently on the exit side of an engine, for enabling operations with taking the radiator exit side liquid temperature into consideration. Within the thermo element of this thermostat, a thermo wax capable of heat expansion is charged, so that the valve body is opened/closed according to the cooling water temperature. Further, the thermo element is combined with a heat generating element such as a nichrome heater to open/close the valve body for electronically controlling the cooling water temperature.
Patent Document 2 discloses a solenoid valve having a movable portion urged to a closing direction by a spring. This solenoid valve is configured to be closed at the time of non-excited state of the coil and opened at the time of the excited state of the coil; thus, switchover between the opened state and the closed state can be effected in a speedy manner. With this, in case the solenoid valve is provided on the engine exit side in the above-described engine cooling system, the switchover between the valve closed state for causing cooling water to circulate via the bypass channel bypassing the radiator and the valve opened state for causing the cooling water to flow through the radiator when the cooling water temperature becomes high, can be effected speedily, thus achieving good valve responsiveness.